Apparatus for cooling and filling liquefied gas transport and storage tanks and improvements in said tanks

ABSTRACT

A transport tank for carrying liquefied gas at about ambient pressure including free-standing inner and outer tank walls formed of material for withstanding low temperatures so that the outer tank acts as a secondary system in the event the inner tank fails and a plurality of interconnecting members coupling the inner tank wall to the outer tank wall so as to limit the relative movement therebetween but provide 2* of freedom within this limit. Stress members secured to the inner and outer tank walls are provided to assure more uniform thermal tank growth in the vertical direction.

[ 1 June 20, 1972 United States Patent Nonnecke Mesnager 5 54] APPARATUS FOR COOLING AND 2,896,271 7/1959 Kloote et al. FILLING LIQUEFIED GAS TRANSPORT 2,959,318 1 1 Cla k et a1. AND STORAGE TANKS AND 2,993,460 7/1961 Dreyer....... IMPROVEMENTS IN S AID TANKS 3,021,027 2/ 1962 Clayton 3,245,571 4/1966 Canty et al.... [72] Inventor: Ernst A. Nonnecke, Hamburg, Germany 3,251,501 5/ 1966 John J. McMullen, Montclair, NJ. FOREIGN PATENTS OR APPLICATIONS May 15, 1968 Related US. Application Data [63] Continuation of Ser. No. 440,081, March 16, 1965.

[73] Assignee:

[22] Filed:

a d a n a C 627,267 9/1961 1,263,560 5/1961 France...

[2!] Appl. No.:

Primary Examiner-Meyer Perlin Attorney-Fleit, Gipple & Jacobson [52] U.S.CL........................................62/45,62/55,220/15, ABSTRACT A transport tank for carrying liquefied gas at about ambient 114/74 .Fl7c 7/00 .62/45, 55; 220/15; 114/74 pressure including free-standing inner and outer tank walls formed of material for withstanding low temperatures so that [58] FieldofSearch.....................

the outer tank acts as a secondary system in the event the 1' ted [56] Re erences Cl inner tank fails and a plurality of interconnecting members UNITED STATES PATENTS coupling the inner tank wall to the outer tank wall so as to limit the relative movement therebetween but provide 2 of freedom within this limit. Stress members secured to the inner and outer tank walls are provided to assure more uniform thermal tank growth in the vertical direction.

12 Claims, 19 Drawing Figures 5560 HHM D006 22207 52223 a 1 Brown Sulfrian Schnelfhardt..........................

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sum 120F12 y puny 93/1609 Q on QP Q Silky/79 Q Q //v VENTOR [7 7255 4. A/mmede ATTORNEYS APPARATUS FOR COOLING AND FILLING LIQUEFIED GAS TRANSPORT AND STORAGE TANKS AND IMPROVEMENTS IN SAID TANKS CROSS REFERENCE TO ANOTHER APPLICATION This application is a Streamlined Continuation of application, Ser. No. 440,081, filed Mar. 16, I965.

The present invention provides an apparatus and method for cooling a transport or storage container to a predetermined low temperature and filling the container with a liquid having a low evaporation point, such as liquefied methane and the like. In a specific embodiment of the present invention, there is disclosed an apparatus and method of the type described used in conjunction with transport tanks on a liquefied methane carrier-ship. The present invention is also drawn to improvements in the structure of the transport tanks of the type described.

For many years, some gases, specifically petroleum gases such as propane and butane, have been transported and stored at ambient temperatures but under appropriate pressure to avoid evaporation. However, during recent years increasing attention has been given to the transportation and storage of liquefied petroleum gases at ambient pressure but at a temperature below its evaporation point. This interest has been brought about by the desire to transport and store liquefied methane, since, as is well known in the art, this gas cannot be economically carried under enormous pressures required to obtain ambient temperature. Therefore, the state of the art has turned toward the development of low temperature containers for use in handling liquefied gases such as methane and the like at preferably ambient pressure.

It is generally accepted that one of the most important commercial problems of carrying liquefied gas below its evaporation temperature is that great time and expense is necessary in cooling insulated double-wall containers of large capacity down to and below the evaporation temperature of the liquefied gas. In the case of methane, this temperature is in the range of l62 C. or 260 F. At the present time, it requires nearly a week to reduce the temperature of an insulated container of the type described to a temperature, for example, of -l 62 C. which, as is readily understood, is commercially expensive from the standpoint of relative inactivity of machinery, men, vessels and the like.

The primary object of the present invention is to provide a method and apparatus for reducing the temperature of the tanks of the type described in a much more efficient, faster and economic manner.

It is another object of the present invention to provide a method and apparatus for filling the tanks of the type described with liquefied gas having a low evaporation temperature in a shorter time than heretofore realized and to increase the safety by reducing the possibility of explosion during introduction of the liquefied gas.

It is yet another object of the present invention to provide improvements in the double-walled transport tank structure of the type described which is incorporated in carrier-ships, the particular structure of the tank being such as to be more structurally sound and safer than tanks heretofore used for this purpose.

Other and further objects of the present invention will become apparent with the following detailed description when taken in view of the attached drawings in which:

FIG. 1 illustrates a liquefied gas carrier-ship having a number of cargo tanks therein.

FIG. 2 is a vertical section taken along line 2-2 of FIG. 1.

FIG. 3 is a horizontal section taken along line 3-3 of FIG. 1.

FIG. 4 is an exploded horizontal section of the corner structure of one of the double-walled tanks of the present invention showing one embodiment of the double-wall stiffeners.

FIG. 5 is a vertical section taken along line 5-5 of FIG. 4.

FIG. 6 is a vertical section of the bottom comer of one of the tanks comprising the present invention.

FIG. 7 is a horizontal section taken along line 7-7 of FIG. 6.

FIG. 8 is a horizontal section of a second embodiment of wall stifi'eners.

FIG. 9 is a side elevation taken along line 9-9 of FIG. 8.

FIG. 10 is a vertical section taken along line 10-1001 FIG. 8.

FIGS. 11, 12, l5, l7, and 18 are schematic diagrams illustrating the apparatus and method of the present invention.

FIGS. l3, 14, 16 and 19 are graphs showing pertinent parameters at various times during the loading and cooling of the liquefied methane tanks.

Referring to the drawings in detail, FIG. 1 illustrates a methane carrier-vessel generally indicated as 10 having four cargo tanks 12 spaced throughout the longitudinal axis of the ship. Each tank 12 extends from the bottom to the top of the hull and has a capacity of 10,000 cubic meters.

As can be seen in FIGS. 2 and 3, the hull 14 of vessel 10 acts as a housing for tank 12 and said tank 12 is supported by the outer foundations l6 and a center foundation l8 fitted between the tank and hull bottoms. Due to the anticipated low temperatures, insulating material 20, such as balsa wood, expanded plastic, polyurethane, batted mineral wool or the like, coats the walls, bottom and top of the hull 14.

Tank 12 comprises an outer corrugated wall 22 and an inner corrugated wall 24 having undulations such that those undulations facing toward each other are aligned and those undulations facing away from each other are aligned. A suitable number of keys (not shown) are mounted between the insulation and the outer wall 22 to enable vertical relative movement therebetween due to thermal expansion and to maintain wall alignment. Inner and outer walls 22 and 24 are spaced from each other for a purpose to be described hereinbelow, and this space will hereinafter be referred to as the wall space. Outer wall 22 is also spaced from the insulation 20, and this space will be hereinafter referred to as the insulation space. Longitudinal bulkhead 26 and transverse bulkhead 28 divide the inner tank into four tank sections, each section having approximately a 10,000 cubic meter volume. Bulkheads 26 and 28 have a number of stifieners (not shown) arranged thereon in the conventional manner.

In order to add additional stiffening to the walls of the tank and also prevent relative movement between walls 22 and24, a suitable number of girders 30 are horizontally mounted at spaced vertical locations around the sides of the tank. As better seen in FIGS. 4 through 7 and in accordance with one principle of the present invention, horizontal stiffeners are welded in the wall space to the outer wall 22 and inner wall 24. The stifi'eners comprise two plates or sections 32 and 34 which face and overlap each other (see FIG. 5) and are held fast by bolts 36. Plates 32 and 34 are shaped to overlap only in the regions where the undulations of walls 22 and 24 are closest; see FIG. 4. Where the undulations are farthest from each other, plates 32 and 34 define an opening 38 of such size to function as a manhole or crawl space so that personnel or instruments can move unimpeded within the wall space for the purpose of conducting safety checks, such as gas leak checks and the like. Moreover, openings 38 enable a free circulation of inert gas more fully described below.

During operation, tank outer wall 22 must be at the same low temperature as inner wall 24 and the cargo being carried in the event inner wall 24 develops a crack or leak and the liquefied methane runs into the wall space; a low-temperature outer wall 22 prevents the liquefied methane from vaporizing, and thus, it reduces the chance of explosion. The overlapping arrangement of stifieners 32 and 34, therefore, serves as safety structure because this arrangement prevents cracks which may develop in inner wall 24 from traveling through the stiffener to the outer wall 22. And in a like manner, cracks developing in outer wall 22, with the structure of the present invention, cannot be transmitted to the inner wall 24. As can be readily seen in FIG. 5, cracks developing in any of the two walls are confined to the respective stifiener associated therewith and are not transmitted through the other stiffener section. For this reason, all connections between walls 22 and 24 are bolted or riveted.

Referring to FIGS. 6 and 7, the stiffener section 34 is integral with the girder 30 extending toward the inside of tank 12. Spaced at suitable horizontal locations and preferably near the bottom of the wall space are l-beam stress members 40 having web sections in a vertical plane and legs 42 and 44 welded to the outer wall 22 and inner wall 24, respectively. The web section of each l-beam 40 is made up of two overlapping plates 46 and 48 secured by rivets or bolts 50 for the same reason as described above for the stiffener sections 32 and .34. I-beam member 40 not only prevents relative vertical and horizontal movement between the inner and outer walls 22 and 24, but it also supplies additional vertical support for inner wall 24. I-beam member 40 is preferably mounted between the walls at locations where the undulations of outer and inner walls face each other and the distance therebetween is the smallest. Similar l-beam members 52 are spaced at suitable horizontal positions between the inner and outer bottom of the tank in the same manner as l-beam member 40. As further describedbelow, the temperature difi'erence between tank top and bottom should preferably not exceed 25 C. so that great stresses from thermal expansion do not appear in walls 22 and 24. If this maximum temperature differential is exceeded, more stress members 40 are needed to prevent structural failure.

FIGS. 8, 9 and illustrate an alternative arrangement for stiffening the inner and outer walls; horizontal girders 30 are welded on the inside of wall 24 as described above. A flexible stiffening member 100 has a base plate 102 welded to the narrow space undulation 24'. Welded on the base plate 102 is a disk 104 having a depression 106 therein. A cup-shaped retain 108 is also welded to plate 102 coaxially with disk 104. A connecting arm 110 having a ball seat at each end bitted within each depression 106 functions to maintain the wall distance. The member 110 also has enlarged ball-shaped ends which cooperate with the inner surface of retainers 108. Pipe 112 is preferably coaxial with assembly 100 and welded to the wall 24 and girder 30.

This flexible stiffener operates to enable slight relative movement (for example, 2 or 3 millimeters) between walls 22 and 24 resulting from, for example, thermal expansion and contraction. Moreover, cracks appearing in one wall will not be transmitted to the other. Fabrication of the tank is also enhanced by this embodiment because the assembly 100 is welded as a unit to one wall; next, the other wall is positioned and the other end of the assembly is welded to the other wall. 7 The stress members 52 located between the bottoms of the double-walled tank need not necessarily be of the flexible assembly 100 because it is anticipated the bottom of the tank will be uniform in temperature distribution unlike the vertical walls 22 and 24 of the tank 12.

At the bottom comers of the tank, there is also provided vertical girders 56 having one end welded to a horizontal girder 30 and its other end acting as a stifiener 34 within the wall space. Elbow-shaped plates 58 and 60 are welded to the horizontal and vertical girders 30 and 56, respectively, to further reinforce and increase the rigidity of the entire tank structure. Flared inserts or shoes 62 are welded between girder 30 and the inward facing undulations of inner wall 24 to increase the base area and spread the supporting forces more uniformly over girder 30.

Each tank section of tank 12 is fitted with one electrodriven submerged pump located on the tank bottom with a capacity of approximately 350 cubic meters per hour. Pumps of this type are well known in the art, and it should operate satisfactorily down to a level of 125 millimeters above the pump section inlet. An equalization gate-valve (not shown) is fitted very closely to the bottom of transverse bulkhead 28 between two tank sections so that the pump of one section can be used as standby pump for the other section. The wall space is provided with one emergency pump to empty that space in the event it becomes necessary, and this pump should have a capacity of approximately 45 cubic meters per hour.

Each tank section is fitted with a filling line, a discharge line, a gas suction line, and an inert gas line, safety valves, vacuum valves (none of which are shown) and any other necessary connections now commonly found on tanks of this type. The safety valves comprise two escape systems, one starboard and one port, and the vacuum valves serve to protect the tanks against undue underpressures. These vacuum valves are connected with a methane pressure system which is held under low overpressure.

Referring now to FIG. 11, the method and apparatus for cooling and filling tanks of the aforementioned type will now be described.

It is well known that liquefied methane should not be poured into a tank which is at ambient temperature, and, for safety reasons, the tank must be cooled down to at least 1 40 C. before introduction of liquefied methane cargo begins.

Before beginning the cooling of the tanks of the type described, the tanks must be purged with an inert for safety reasons. For this purpose, a quantity of nitrogen is generated by a nitrogen-generating plant located on shore or on the vessel, and the gas is stored under pressure in large tanks.

Any suitable apparatus can be used to purge the spaces and tank with inert gas. For example, pipes can be installed running through the insulation space to the bottom thereof and communicate with the wall space at the bottom through the outer gas-tight wall 22. Openings at the bottom of the portion of the pipe within the insulation space enable introduction of gas therein. A separate pipe from the inert gas source communicates with the inner tank. Appropriate collecting manifolds are mounted at the top of the insulation and wall spaces and deliver the inert to a blower.

Another arrangement provides two pipes each extending through the insulation and wall space, respectively, with openings at the bottoms thereof.

It should be understood that any suitable arrangement can be used as long as the inner and outer walls 22 and 24 are maintained gas tight.

Before filling the insulation and wall space with inert gas, the air within these spaces is circulated two or three times over the dehydration units to reduce the moisture content thereof. This procedure prevents condensation from forming during later operations.

After dehydration, the inert gas in this example, nitrogen is delivered at zero degrees centigrade from the storage tanks to the inner tank, insulation space from top to bottom and up the wall space of the cargo tank to a blower and out an exhaust until thespaces and tank are purged. At this time, the nitrogen gas is at 0 C, and within two or three volume exchanges, the tank, wall space, and insulation space are uniformly cooled to about 0 C. This step takes approximately 5 hours for one tank having a capacity of 10,000 cubic meters. After purging, the exhaust is closed, and the nitrogen recirculated to the heat exchanger.

After purging is completed, the cooling of the tanks begins. The heat exchanger is first fed from a source of liquefied gas (in this example, methane) wherein the nitrogen assumes a lower temperature than during inerting mentioned above. Again, the nitrogen gas is circulated by a blower to the tank insulation space and between the wall space so that the inner and outer walls of the tank 22 and 24, respectively, are cooled in a uniform manner. At the same time, the methane exhausting from the heat exchanger, although now a vapor, has a temperature much below the ambient, and this vaporized methane is fed through pipes and subsequently released directly into the tank to cool the interior thereof. The rising methane gas within the tank is collected and fed to heater where it is heated to approximately 15 C. and then supplied to a gas turbine or'a fuel storage bin. Ifthis collected methane gas is not needed for fuel, it is recirculated to a cooling unit on shore where it is again converted to liquefied gas and fed to the main source tank. To speed the lowering of the tank temperature, a small amount of liquefied gas, such as methane, is also sprayed during this time directly in the tank.

The blower and the heat exchanger capacities are preferably set in such a way that at the beginning of the cooling procedure the difference in temperature between the tank top and tank bottom does not exceed a maximum value of 25 C. for safety reasons and the aforementioned structural reasons.

After a cooling period of approximately 65 hours, the insulation space and wall space and tank temperature is roughly 130 C. Once the tank temperature reaches -1 30 C., it is anticipated the rate of lowering the tank temperature could be speeded up in any number of ways.

One method of reducing the temperature of the tank and the insulation and wall spaces even further is to feed liquid nitrogen, which has an evaporation temperature of approximately 1 90 C. to the heat exchanger in place of the liquefied methane. The circulation of the nitrogen gas continues for approximately another -hour period after which the tank bottom reaches a temperature of l40 C. With the temperature of the insulation space, the wall space, the tank and the tank sidewalls at approximately 140 C., the liquefied methane is then fed through the fill line directly into the storage tanks.

Another way to reduce the tank temperature from 130 C. is to change from a heat exchanger having 100 square meter surface area to another heat exchanger having a considerably greater surface area.

The method of the present invention enables four 10,000 cubic metric tanks to be cooled in approximately 80 hours total time from the initial dehydration step to the final filling of the tank with liquefied methane. It is important to note that the insulation space, the wall space, the inner and outer walls 22 and 24, and the inner tank are cooled at the same uniform rate due to the passing of the cooled nitrogen gas between the insulation space and the wall space of the tank.

After the tank has been completely filled with liquefied methane, its low temperature is maintained by evaporation; during boil off, the gas is collected and can be fed as fuel to appropriate gas storage areas of the vessel. It has been found that a cargo of 10,000 tons yields about 30 to 40 tons of boil ofi' per day, which is a sufficient quantity to efficiently propel the vessel. According to the present invention, the pumps located in the bottom of the tanks deliver a small amount of liquefied methane to the heat exchanger as the nitrogen is circulated within the insulation and wall spaces in order to keep the circulating nitrogen at about l45 C.

After the vessel reaches its destination and the cargo is unloaded down to ballast condition, the remaining liquefied methane is kept cool and the aforementioned 25 C. temperature differential is maintained by the circulation of nitrogen through the heat exchanger with pumped liquefied methane as the medium. The liquefied methane is delivered to the top of the tank through pipes after it exhausts from the heat exchanger and is sprayed into the tank to maintain the top of the tank within the aforementioned temperature differential. It is commonly known that balast trips are somewhat dangerous, and the aforementioned recirculation of nitrogen and methane vapors also acts as a safety device to prevent explosion and rapid evaporation. It should be understood that any suitable media can be used in the heat exchanger during cargo and ballast trips, and any suitable liquefied gas can be stored on board under pressure or a liquid nitrogen generator can be provided on board for this purpose.

The following is but one example of the present invention to be conducted on a 10,000 cubic meter tank (surface area between tank walls: 1,500 m total surface area of nitrogen space is 6,800 m').

A nitrogen generator is used to extract nitrogen from the atmosphere and deliver the same in liquid form to large storage tanks on shore. Dehydration of the insulation and the insulation space is effected by circulating three volume exchanges of air in the insulation and wall spaces of two hydration units. For inerting, liquid nitrogen is fed from shore to a heat exchanger on the vessel where it is vaporized and subsequently fed to the insulation space at about 0 C. The entire tank, insulation and wall spaces are purged by use of a blower (capacity: 60,000 m lhr.) mounted adjacent the tank drawing from these spaces and circulating the air out an exhaust. This flushing of tank and spaces continues for two or three volume exchanges, approximately 5 hours. When all but nitrogen is removed from these spaces, the exhaust is closed, and the blower then feeds the heat exchanger and the source of liquid nitrogen is shut off. In this way, the insulation, wall spaces, blower, heat exchanger and connecting lines define a closed circuit filled with nitrogen gas in continuous circulation.

Next, liquefied methane is fed to the heat exchanger from large storage tanks on shore and used as a sink for the circulating nitrogen gas. When the methane is first (time zero) introduced to the heat exchanger, the blower is advanced to circulate the nitrogen at 40 exchanges per hour. The heat exchanger has an area of about 100 m and the nitrogen is rapidly cooled. See FIG. 13. The methane exhausting the heat exchanger is in vapor form, and it is returned to shore to collecting tanks to be reliquefied. The heat exchanger and blower are set so that the temperature diflerence between the top and bottom of the tank is not more than 25 C. See FIG. 14. Afler a period of about 20 hours, the condition of the tank is that iilustrated by FIG. 15.

Approximately six tons of liquefied methane is sprayed directly into the tank (see FIG. 16) over a period of 75 hours beginning at the fifth hour or when the tank wall temperature is about 20 C. When the tank wall and methane vapor temperatures are within approximately 10 C. of each other, the input of liquefied methane should be gradually reduced to zero over a 20-hour period (see FIG. 17, summation of liquefied methane, from hours 30 to 50).

After 65 hours, the tank and gas temperatures are practically the same so that the tank temperature will no longer rapidly lower due to inherent heat absorption thereof. At this u'me, liquid nitrogen is fed to the heat exchanger in place of liquefied methane. FIG. 13 illustrates the respective evaporation temperatures of nitrogen and methane. At the same time, liquefied methane is again fed directly into the tank at a rapid rate (see FIG. 16) until the desired tank temperature is reached.

Liquid nitrogen is fed to the heat exchanger for a period of about 15 hours until the nitrogen gas and tank cool to about -l40 C. For the tank to reach this temperature in about hours, approximately 82 tons of methane and 36 tons of nitrogen are vaporized. After 80 hours, the distribution of temperatures is that illustrated by FIG. 18.

On a ballast trip, the nitrogen gas is kept moving at a rate of 20 exchanges per hour whereby the difference between top and bottom tank temperature is not greater than 25 C. During the ballast trip, 46 tons of liquefied methane per hour is delivered through the heat exchanger to maintain the nitrogen at its low temperature of -14l C., and the methane from the heat exchanger is sprayed directly back into the tank. See FIG. 12. Under loaded conditions, the liquefied methane from the heat exchanger is heated to 15 C. and fed to the boiler as fuel.

It is readily understood that other and further modifications can be made without departing from the spirit and scope of the present invention.

I claim:

1. In combination, a tanker having a cargo hold of predetermined shape and an insulated container for liquefied gases maintained at atmospheric pressure and cryogenic temperature mounted within said hold, said container including:

a. a primary tank substantially similar in shape to said hold,

b. a larger secondary tank substantially similar in shape to said hold and surrounding said primary tank,

c. thermally conductive rigid structural means for maintaining said primary tank in fixed, spaced relation within said secondary tank and for reducing the thermal gradient between said primary and secondary tanks when said liquefied gases are initially loaded into said container and during transportation of said gases in said tanker,

- d. said tanks providing primary and secondary liquid-tight barriers for retaining cryogenic liquefied gas cargo at atmospheric pressure,

e. foam insulation means applied externally and secured about said secondary tank in an enveloping relation therewith, said insulation means constituting substantially the sole insulation means of said container,

f. a longitudinal bulkhead within said container dividing said container in substantially liquid-isolated port and starboard portions,

g. a transverse bulkhead dividing each of said port and starboard portions into fore and aft portions, and

h. separate submerged pump means located in each of said port and starboard portions for pumping liquefied gases from said container.

2. The combination of claim 1 including cooperative key and keyway means between the exterior of said container and the interior of said cargo hold to permit relative movement therebetween due to thermally induced changes in the dimensions of said container.

3. In combination, a tanker having a cargo hold of predetermined shape and a container for liquefied gases maintained at atmospheric pressure and cryogenic temperature mounted within said hold, said container including, an internal longitudinal bulkhead dividing said container in substantially liquid isolated port and starboard portions, a transverse bulkhead dividing each of said port and starboard portions into fore and aft portions, a plurality of apertured webs extending from side to side of said container, a primary tank substantially similar in shape to said hold and secured to the periphery of said webs and periphery of each said bulkhead, a plurality of stiflening structural members secured to the exterior of said primary tank for rigidifying said tank, and a larger secondary tank substantially similar in shape to said primary tank secured to said structural members in spaced relation to said primary'tank, said structural members in combination with each said bulkhead and said webs providing a common structural cage for said primary and secondary tanks, and insulation means secured to the outer surface of said secondary tank.

4. The combination of claim 3 including cooperative key and keyway means between the exterior of said container and the interior of said cargo hold to permit relative movement therebetween due to thermally induced changes in the dimensions of said container.

5. A tank arranged in a vessel for transporting at extremely low temperatures a liquefied gas such as methane and the like, said tank comprising an inner wall and an outer wall spaced from and enclosing said inner wall and defining a wall space therebetween, said inner and outer walls being corrugated and having undulations arranged so that the undulationsprojecting towards the wall space are aligned and the undulations projecting away from the wall space are aligned, a plurality of girders connected to said inner wall and each having an inner edge spaced inward from said inner wall, each said girder extending through said inner wall and having a terminating edge located in said wall space, a stiffening plate associated with each of said girders and being arranged parallel to its respective girder, each said stiffening plate extending through the outer wall of said tank and having an outer edge terminating near the outer wall, each said girder and its associated stiffening plates being arranged perpendicular to the longitudinal axes of the corrugations of said inner and outer walls, said stiffening plate having an inner edge located in the wall space, a part of said stiffening plate contacting and overlapping a part of its respective girder, and mechanical means securing in rigid engagement the overlapping part of said stifi'ening member and its girder, whereby rigid support of said inner and outer tank walls is afforded by said girder and stiflening member, and cracks developing in one of said walls are prevented from travelling to the other of said walls, and wherein said tank has four vertical sides, each corrugation of said inner and outer walls of each side extending vertically and wherein at least one of said girders is arranged in a horizontal plane, the overlapping parts of said girder and stifiening plate being located between each pair of corrugations which project toward the wall space, and the parts of said girder and stiffening plate between the pairs of con'ugations that project away from the wall space being cut to define an enlarged opening, and said tank further comprising stress members mounted between said inner and outer walls of the vertical sides of said tank near the bottom thereof, each stress member comprising a first and second overlapping, vertically extending plate, said first plate being integrally mounted on and perpendicular to one corrugation of said inner wall which projects toward the wall space, and said second plate being integrally mounted on and perpendicular to the corrugation of said outer wall which is aligned with said one corrugation, and mechanical means securing said first and second plates in rigid engagement.

6. A tank as set forth in claim 5 wherein said corrugations to which said stress member is mounted project toward the wall space.

7. A tank as set forth in claim 5 wherein said mechanical means comprises bolts and said openings are of sufficient size to enable passage of a man between the undulations that project away from each other.

8. A transport tank for carrying liquefied gas at about ambient pressure, said tank having an outer wall and a spaced inner wall, said walls comprising material capable of withstanding the low cargo temperatures so that said outer wall acts as a secondary or back-up barrier in the event of failure of the inner wall, an interconnecting member is provided between the outer and inner walls to limit the relative movement therebetween, said interconnecting member comprising a pair of base members aligned generally perpendicular to their planes, one base member being mounted on the outside of the inner wall and the other being mounted on the inside of the outer wall, an interconnecting arm means having each end rotatably retained by one of said base members for transmitting tension and compression forces and affording within limits two degrees of freedom between said pair of base members, a girder located inside the inner wall and welded thereto to reinforce the same, the plane of said girder being aligned with said interconnecting member, and a pipe arranged coaxial with said interconnecting member having one end mounted on the inside of said inner wall and its other end being connected to said girder for transmitting force loads to and from the aligned interconnecting member.

9. A transport tank in combination with a ship for storing liquefied gas at about ambient pressure, said tank having a free standing outer wall and a spaced free standing inner wall to form a free space therebetween, said walls being formed of material capable of withstanding the low cargo temperature, said tank arranged within the ship's hull and thennal insulation being disposed between the tank and ship surrounding structure, said tank comprising stress members to prevent relative movement and unequal local thermal growth between the outer and inner wall due to thermal expansion and contraction, each such stress member being located within the space between said outer and inner wall and having a pair of mounting plates one of which is secured to the inside of the outer wall and the other of which is secured to the outside of the inner wall, said stress member further comprising a web mounted perpendicularly between and connected to said mounting plates, said web extending vertically of the tank.

10. The combination as set forth in claim 9, wherein said web comprises two overlapping plates connected by mechanical means, such as bolts or the like, one of said plates being mounted to one of said mounting plates and the other of said plates mounted to the opposite mounting plate.

11. The combination as set forth in claim 10, wherein a plurality of stress members are provided near the bottom of the upwardly extending walls of said tank and whereinthe vertical dimension of said web is greater than the horizontal dimension thereof.

12. A transport tank in combination with a ship for transporting a liquefied gas at about ambient pressure, said tank having a free standing outer walland a spaced free standing inner wall to form a free space therebetween, thermally insulated foundations spaced from one another and located in the hold of said ship supporting said tank, and a primary structural tank framework comprising bottom girders connected to said inner and outer tank walls along the bottoms thereof to provide primary support for said tanks, said bottom girders being vertically aligned with said foundations to transmit the primary force loads directly to said foundations. 

1. In combination, a tanker having a cargo hold of predetermined shape and an insulated container for liquefied gases maintained at atmospheric pressure and cryogenic temperature mounted within said hold, said container including: a. a primary tank substantially similar in shape to said hold, b. a larger secondary tank substantially similar in shape to said hold and surrounding said primary tank, c. thermally conductive rigid structural means for maintaining said primary tank in fixed, spaced relation within said secondary tank and for reducing the thermal gradient between said primary and secondary tanks when said liquefied gases are initially loaded into said container and during transportation of said gases in said tanker, d. said tanks providing primary and secondary liquid-tight barriers for retaining cryogenic liquefied gas cargo at atmospheric pressure, e. foam insulation means applied externally and secured about said secondary tank in an enveloping relation therewith, said insulation means constituting substantially the sole insulation means of said container, f. a longitudinal bulkhead within said container dividing said container in substantially liquid-isolated port and starboard portions, g. a transverse bulkhead dividing each of said port and starboard portions into fore and aft portions, and h. separate submerged pump means located in each of said port and starboard portions for pumping liquefied gases from said container.
 2. The combination of claim 1 including cooperative key and keyway means between the exterior of said container and the interior of saId cargo hold to permit relative movement therebetween due to thermally induced changes in the dimensions of said container.
 3. In combination, a tanker having a cargo hold of predetermined shape and a container for liquefied gases maintained at atmospheric pressure and cryogenic temperature mounted within said hold, said container including, an internal longitudinal bulkhead dividing said container in substantially liquid isolated port and starboard portions, a transverse bulkhead dividing each of said port and starboard portions into fore and aft portions, a plurality of apertured webs extending from side to side of said container, a primary tank substantially similar in shape to said hold and secured to the periphery of said webs and periphery of each said bulkhead, a plurality of stiffening structural members secured to the exterior of said primary tank for rigidifying said tank, and a larger secondary tank substantially similar in shape to said primary tank secured to said structural members in spaced relation to said primary tank, said structural members in combination with each said bulkhead and said webs providing a common structural cage for said primary and secondary tanks, and insulation means secured to the outer surface of said secondary tank.
 4. The combination of claim 3 including cooperative key and keyway means between the exterior of said container and the interior of said cargo hold to permit relative movement therebetween due to thermally induced changes in the dimensions of said container.
 5. A tank arranged in a vessel for transporting at extremely low temperatures a liquefied gas such as methane and the like, said tank comprising an inner wall and an outer wall spaced from and enclosing said inner wall and defining a wall space therebetween, said inner and outer walls being corrugated and having undulations arranged so that the undulations projecting towards the wall space are aligned and the undulations projecting away from the wall space are aligned, a plurality of girders connected to said inner wall and each having an inner edge spaced inward from said inner wall, each said girder extending through said inner wall and having a terminating edge located in said wall space, a stiffening plate associated with each of said girders and being arranged parallel to its respective girder, each said stiffening plate extending through the outer wall of said tank and having an outer edge terminating near the outer wall, each said girder and its associated stiffening plate being arranged perpendicular to the longitudinal axes of the corrugations of said inner and outer walls, said stiffening plate having an inner edge located in the wall space, a part of said stiffening plate contacting and overlapping a part of its respective girder, and mechanical means securing in rigid engagement the overlapping part of said stiffening member and its girder, whereby rigid support of said inner and outer tank walls is afforded by said girder and stiffening member, and cracks developing in one of said walls are prevented from travelling to the other of said walls, and wherein said tank has four vertical sides, each corrugation of said inner and outer walls of each side extending vertically and wherein at least one of said girders is arranged in a horizontal plane, the overlapping parts of said girder and stiffening plate being located between each pair of corrugations which project toward the wall space, and the parts of said girder and stiffening plate between the pairs of corrugations that project away from the wall space being cut to define an enlarged opening, and said tank further comprising stress members mounted between said inner and outer walls of the vertical sides of said tank near the bottom thereof, each stress member comprising a first and second overlapping, vertically extending plate, said first plate being integrally mounted on and perpendicular to one corrugation of said inner wall which projects toward the wall space, and said second plate being intEgrally mounted on and perpendicular to the corrugation of said outer wall which is aligned with said one corrugation, and mechanical means securing said first and second plates in rigid engagement.
 6. A tank as set forth in claim 5 wherein said corrugations to which said stress member is mounted project toward the wall space.
 7. A tank as set forth in claim 5 wherein said mechanical means comprises bolts and said openings are of sufficient size to enable passage of a man between the undulations that project away from each other.
 8. A transport tank for carrying liquefied gas at about ambient pressure, said tank having an outer wall and a spaced inner wall, said walls comprising material capable of withstanding the low cargo temperatures so that said outer wall acts as a secondary or back-up barrier in the event of failure of the inner wall, an interconnecting member is provided between the outer and inner walls to limit the relative movement therebetween, said interconnecting member comprising a pair of base members aligned generally perpendicular to their planes, one base member being mounted on the outside of the inner wall and the other being mounted on the inside of the outer wall, an interconnecting arm means having each end rotatably retained by one of said base members for transmitting tension and compression forces and affording within limits two degrees of freedom between said pair of base members, a girder located inside the inner wall and welded thereto to reinforce the same, the plane of said girder being aligned with said interconnecting member, and a pipe arranged coaxial with said interconnecting member having one end mounted on the inside of said inner wall and its other end being connected to said girder for transmitting force loads to and from the aligned interconnecting member.
 9. A transport tank in combination with a ship for storing liquefied gas at about ambient pressure, said tank having a free standing outer wall and a spaced free standing inner wall to form a free space therebetween, said walls being formed of material capable of withstanding the low cargo temperature, said tank arranged within the ship''s hull and thermal insulation being disposed between the tank and ship surrounding structure, said tank comprising stress members to prevent relative movement and unequal local thermal growth between the outer and inner wall due to thermal expansion and contraction, each such stress member being located within the space between said outer and inner wall and having a pair of mounting plates one of which is secured to the inside of the outer wall and the other of which is secured to the outside of the inner wall, said stress member further comprising a web mounted perpendicularly between and connected to said mounting plates, said web extending vertically of the tank.
 10. The combination as set forth in claim 9, wherein said web comprises two overlapping plates connected by mechanical means, such as bolts or the like, one of said plates being mounted to one of said mounting plates and the other of said plates mounted to the opposite mounting plate.
 11. The combination as set forth in claim 10, wherein a plurality of stress members are provided near the bottom of the upwardly extending walls of said tank and wherein the vertical dimension of said web is greater than the horizontal dimension thereof.
 12. A transport tank in combination with a ship for transporting a liquefied gas at about ambient pressure, said tank having a free standing outer wall and a spaced free standing inner wall to form a free space therebetween, thermally insulated foundations spaced from one another and located in the hold of said ship supporting said tank, and a primary structural tank framework comprising bottom girders connected to said inner and outer tank walls along the bottoms thereof to provide primary support for said tanks, said bottom girders being vertically aligned with said foundations to transmit the primary force loads directly to said foUndations. 